Hillary Clinton has said she wants the UK to stay in the European Union, adding her voice to that of US President Barack Obama in backing the Remain campaign. The front-runner for the Democratic party’s presidential nomination is favoured to become the first female US Commander-in-Chief, which would make her a key diplomatic partner for the UK and EU, whether or not British voters choose Brexit in June.

In a statement to The Observer, Ms Clinton’s senior policy adviser, Jake Sullivan, said: “Hillary Clinton believes that transatlantic cooperation is essential, and that cooperation is strongest when Europe is united. She has always valued a strong United Kingdom in a strong EU. And she values a strong British voice in the EU.”

On his final presidential visit to the UK this week, Mr Obama made a strong case for the country to stay in Europe, warning Brexit would put the British at “the back of the queue” when it came to negotiating trade agreements. His comments drew criticism from the Leave camp, including London Mayor Boris Johnson, who wrote that it was “hypocritical” of the US to urge the UK to “surrender control of so much of our democracy”.

Ms Clinton’s husband, former US president Bill Clinton, has also said he hopes the UK remains part of the EU. Receiving an award for his part in the Northern Ireland peace process last month, Mr Clinton said Northern Ireland would “really get whacked” if Britain withdraws from the EU. “I hope they don’t,” he said, adding: “It’s too easy to believe that the only solution to the problems in the world is to hunker down.”

Ms Clinton and her husband were campaigning in at least three US states between them this weekend, ahead of Tuesday’s primaries in Pennsylvania, Maryland, Connecticut, Delaware and Rhode Island. The former Secretary of State holds a comfortable poll lead in the two that hold the most delegates, Pennsylvania and Maryland.